Many separation and reaction processes involving liquid and/or gaseous reagents require contact with a solid surface. In some cases, the solid surface provides physical sites where one or more of the steps involved in the process can occur. Examples are dehydration of ethanol with an alumina catalyst and the cracking of petroleum with molybdenum oxide catalysts. In other cases, the solid surface can support finely divided catalytic particles which are not self supporting. Examples are platinum or palladium on the surface of alumina or silica, for the catalysis of hydrogenation reactions, and iron on silica to catalyze Fischer-Tropsch reactions. The support helps to enhance the performance of the catalyst through increased dispersion. In some cases a catalyst/support interaction effect also influences the performance of the catalyst.
Solid supports are also used to support biological agents such as enzymes so that they can act upon a liquid or gaseous material.
Many physical and chemical adsorption processes also rely on materials with high surface areas for the removal of one or more of the constituents of a fluid stream. Removal of trace chemicals and water vapour from air by molecular sieves or activated carbons is an example.
In some other cases solids are used as a contact medium to promote contact between phases. For example ceramic rings are used to increase the mass transfer rates between gaseous and liquid streams in mass transfer operations.
The solid materials used to catalyze, support a catalyst or chemical or biological agent, or adsorb gas or liquid molecules or act as a contact medium will be called generically, in this disclosure, "support materials".
In the processes set out above, it is important for the support materials to have high surface areas in order to be effective for their functions. The high surface area in created by pores in the particles. When the support materials are used in their fine particle forms in packed beds, fluids cannot pass easily through the beds, leading to excessive pressure drops across such beds. Furthermore, the fine particles get entrained in the fluid stream and leave the vessel, necessitating fines removal applications downstream.
To avoid such pressure drops, it is known to deposit small particles of support materials onto carriers made of an inert material such as a ceramic. However, this has the disadvantage that much of the volume through which the fluid flows is then filled with an inert ceramic carrier, and thus the effective action of the support material per unit volume of the reaction vessel is reduced.
It has been recognized that one way of avoiding these problems is to agglomerate the particles of support material into larger shaped forms. If this could be done in such a way so as to keep a large surface area, the pressure drop problem would be reduced. However, most particles of support material do not have sufficient strength to be molded into shaped articles and to retain the moulded shapes while retaining high surface area. Sintering can be done in some cases, by using high temperature or high pressure, but this has the undesirable effect of reducing the porosity of the material by packing the particles tightly together, and the heat or pressure may also destroy the valuable properties of the material. It is possible to glue the particles together in some cases with an adhesive, but the adhesive itself covers an appreciable portion of the surface area, and can reduce the desirable properties of the material.
There are many materials in the form of fine particles which cannot be agglomerated into larger articles by any of the above conventional methods, without losing the desirable properties of such particles. For example, fine beads of styrenedivinylbenzene is one of these materials. On heating, this material decomposes, and forming by high pressure compaction or adhesive bonding causes a loss of properties.
One substance that has been used with some success to carry a catalyst is polytetrafluoroethylene (PTFE) which is manufactured for example by E.I. du Pont de Nemours & Co. under the trade mark "TEFLON". U.S. Pat. No. 4,025,560, (Rolston et al), shows the use of PTFE cubes as a catalyst support, with the catalyst on the exterior of the cubes. It has also been proposed to blend PTFE with polycarbonmonofluoride (as is shown in Japanese published application 94346/1982 (Okito)) or with up to 15% of a styrene polymer or a styrenedivinylbenzene copolymer or with fluorocarbon, as is shown in Canadian Patent 1,124,416 (Nakane et al). In the Nakane patent, the PTFE is blended with fluorocarbon or styrene(co)polymer and is then blended with a lubricant, extruded and stretched and treated with a platinum-containing agent to introduce platinum into the product. The stretching is a normal part of the treatment of PTFE compounds as such compounds are greatly increased in tensile strength by it.
While materials made by the Rolston, Okito and Nakane methods may have utility in some areas, they do not have a high ratio of surface area to volume. Therefore, in reactions where the materials support a catalyst or act as a contact medium, a large volume of such materials must be used in order to carry out reactions within a reasonable time and at a reasonable flow rate. This tends to make the use of such materials uneconomic.